1. Field of the Invention:
Epoxy resin advancement involves the reaction of a polyepoxide such as a polyglycidyl ether of an aromatic diol, for example, bisphenol A, with a polyfunctional material which is reactive with the epoxide groups. Examples of such polyfunctional material are those which contain at least two active hydrogens per molecule such as ##STR1## Specific examples of such materials are aromatic diols, polymeric glycols such as polyether diols and polyester polyols and polymercapto compounds.
The epoxy resin advancement is not a curing or crosslinking reaction but rather a chain extension reaction resulting in a higher molecular weight, ungelled reaction product which provides for better properties than the lower molecular weight precursor.
2. Brief Description of the Prior Art:
U.S. Pat. Nos. 4,468,307 and 4,104,147 disclose epoxy resin advancement with certain polyether and polyester polyols, respectively. The resultant advanced epoxy resins can be further reacted with cationic salt group formers to prepare cationic resins which are useful in the electrodeposition process. Chain extension of the polyepoxides with these polymeric polyols improves many physical properties associated with the resultant cationic electrodeposition coating.
One problem associated with polymeric polyol chain extension, however, is competing reactions. Under chain extension reaction conditions which are usually in the presence of amine catalyst, epoxy-epoxy reaction and epoxy-secondary hydroxyl reactions compete with the desired polymeric hydroxyl-epoxy reaction. This can result in considerable chain branching of the polymeric backbone leading to undesirably high molecular weight products which can adversely affect the smoothness of the resultant electrodeposited coating. Also, these competing reactions result in unreacted polymeric polyol chain extender which can be deleterious to the resultant coating composition.
These problems can be significantly minimized by chain extending with a polymeric mercapto compound such as disclosed in U.S. Patent 4,260,720. Under chain extension reaction conditions, the mercapto epoxy reaction proceeds relatively rapidly compared to the competing side reactions resulting in a resin which has better properties and does not have much unreacted chain extender. Unfortunately, the mercapto compounds have undesirable odor problems associated with them making them very difficult to work with.
In accordance with the present invention, it has been found that hydroxyalkyl-substituted urethanes containing at least two active hydrogens per molecule, preferably poly(hydroxyalkyl)-substituted urethanes in which the hydroxy groups are in the beta position to the urethane group make excellent chain extenders for polyepoxides, particularly polyglycidyl ethers of polyphenols. These materials result in an epoxy resin advancement with minimal competing reactions. It is believed the reaction product is essentially linear and there is essentially no unreacted chain extender in the reaction product. The resulting reaction products, when further modified to include cationic salt groups, can be used as resinous film formers in the process of cationic electrodeposition where they result in the formation of exceedingly smooth electrodeposited coatings.